Belief or believing, in simplest terms, is the acceptance by the mind of something as true. A belief is something which is believed. One can believe something to be true without having knowledge that it is true. Also, one can believe something to varying degrees of certainty (see also Probability).

Calling someone a believer often connotes belief in a god, especially the Christian God (in the U.S., anyway). Sometimes, though, the belief is related to some other supernatural or unsupported claim, such as ESP or alien abduction — or even to a philosophical position or political opinion.

Degrees of belief

Common phrases describing ones own degree of belief:

I know that

I believe that

I think that

I feel that

I [would] guess that

I [would] bet/wager that

I [would] like to think/believe that

I have no reason to doubt that

I have no reason not to believe/think that

Common phrases describing the degree of believability of a statement of fact:

It is true

It is certainly true

It is surely true

It is almost certainly/surely true

It is most likely true

It is likely true

It is probably true

It is possibly true

It is perhaps true

It may be true

It might be true

It should be true

It can/could be true

In addition, most of the above statements could be negated in two different ways, giving rise to different connotations. For example, "It is not likely [to be] true" is different from "It is likely not [to be] true".

Belief vs. faith

Faith is sometimes defined as belief without evidence, or in the face of contradictory evidence. Clearly, "belief" does not always imply "faith," as one can believe something based on evidence or for other rational reasons (e.g., logical arguments). For example, one does not need to have faith that the sun will rise tomorrow, since the fact that the sun rises every day is supported by a lifetime of personal experience, thousands of years worth of recorded observations, and a scientific model of the earth's rotation on an axis.

Belief in

However, the definition of "faith" given above is usually not what religious people mean when they use the word: to them, having faith is closer to "trusting" than simply "believing" (see the Faith article for more information). The phrase "belief in" is sometimes used to imply this element of trust. For example, when Christians say they "believe in" God, they usually don't simply mean that they believe he exists, but that they believe he loves them, has a plan for their lives, will see them through hard times, etc. For this reason, when Christians ask what atheists "believe in", or whether an atheist "believes in" evolution or the big bang, the best response is likely to include an explanation of the difference between belief and faith.